Carbon dioxide is a greenhouse gas produced by natural processes and everyday human activities, especially the burning of fossil fuels. The Keeling Curve is a measurement of the concentration of carbon dioxide in the atmosphere made atop Hawaii’s Mauna Loa since 1958. It is the longest-running such measurement in the world. The Scripps CO2 program was initiated in 1956 by Charles David Keeling and operated under his direction until his death in 2005. It is currently being continued by Ralph F. Keeling, who also runs a parallel program at Scripps to measure changes in atmospheric oxygen abundances. Carbon dioxide measurements at Mauna Loa are also being made by an independent instrument operated by NOAA.
By the early 1970s this curve was getting serious attention, and played a key role in launching a research program into the effect of rising CO2 on climate. Since then, the rise has been relentless and shows a remarkably constant relationship with fossil-fuel burning, and can be well accounted for based on the simple premise that 57 percent of fossil-fuel emissions remain airborne.
The Mauna Loa record can now be placed in the context of the variations in CO2 over the past 400,000 years, based on reconstructions from polar ice cores. During ice ages, the CO2 levels were around 200 ppm, and during the warmer interglacial periods, the levels were around 280 ppm.
Looking ahead, if the rate of fossil-fuel burning continues to rise on a business-as-usual trajectory, such that humanity exhausts the reserves over the next few centuries, CO2 will continue to rise to levels of order 1500 ppm. The atmosphere will not return to pre-industrial levels even tens of thousands of years into the future. Unless serious efforts are made to reduce the dependence on fossil fuels, it is clear that we are on a threshold of a new era of geologic history, one with climate very different from that of our ancestors. These curves not only demonstrate the seriousness of the global warming problem, but also illustrate the power of continuous time series to communicate and clarify the essential science.